A striking feature of higher cells, from yeast to human cells, is their compartmental organization into membrane-bound organelles, each with a unique composition and function. The overall goal of our research program is to understand how the cell assembles the organelles of the secretory pathway, particularly the Golgi complex, a multi-compartment organelle that plays an essential role in the transport, modification and sorting of proteins. Towards this goal, the mechanisms by which a protein (Mnnlp) is localized to the Golgi complex will be studied. The yeast Saccharomyces cerevisiae will be used for this work because of the fundamental similarity of the secretory pathway in all eukaryotic cells coupled with the powerful genetic and molecular approaches available to the yeast researcher make this organism an ideal model system. The proposed studies stem from the recent finding that members of a mitogen-activated protein (MAP) kinase family are required for Mnn1p Golgi localization. The specific aims of this proposal are to: 1) Define the role that the HOG MAP kinase pathway plays in controlling the compartmental organization of the yeast Golgi complex. 2) Identify proteins outside of the HOG pathway that are required for Mnnlp Golgi localization. 3) Analyze the mechanism of Mnnlp Golgi localization. 4) Define the role of COPII vesicles in anterograde protein transport through the Golgi complex. It is anticipated that these studies will result in a more detailed view of the compartmental structure of the Golgi complex, the constituent proteins required to maintain the structure and function of this organelle, and the mechanisms the cell uses to localize proteins to the Golgi complex. There are profound implications to human health that can be derived from this study. Innumerable proteins such as growth factors and their receptors, proteins of the immune system and viral proteins, all depend on the Golgi complex to be transported through the cell, to be sorted to the appropriate subcellular domain, and to be posttranslationally processed to their final chemical form. Cell-cell and cell-matrix interactions important for normal development, and many cell-pathogen interactions all involve carbohydrate epitopes formed in the Golgi by glycosyltransferases. In addition, loss of specific Golgi enzyme activities lead to several serious human disorders including I- cell disease, pseudo-Hurler polydystrophy, HEMPAS disease and Lowe syndrome.